Trichodesmium: The world's most famous nitrogen fixer

Editor's Note: Journalist and crew member Kathryn Eident and scientist Jeremy Jacquot are traveling on board the RV Atlantis on a monthlong voyage to sample and study nitrogen fixation in the eastern tropical Pacific Ocean, among other research projects. This is the sixth blog post detailing this ongoing voyage of discovery for ScientificAmerican.com.

Imagine you’re in space, floating high above the Earth. Picture the world’s oceans, glimmering sapphire under the heat of the sun and the protection of the ozone layer. Look closer, there’s a patch of brown in the middle of all that blue. It’s a bloom of phytoplankton called Trichodesmium, a “world famous” nitrogen fixer.

Shaped like pieces of human hair or filament, tricho floats on the ocean’s surface in huge brownish patches called colonies, which are sometimes large enough to be seen by satellites in space. Their slender shape has earned these multicellular, photosynthesizing creatures the nickname, “saw dust of the sea.”

Trichodesmium, a genus of cyanobacteria, packs a lot of punch in the nitrogen fixation world; in fact, they’re one of the most well-known nitrogen fixers. They have a special enzyme, called nitrogenase, to break gaseous nitrogen’s tough triple bond and convert it into a form other creatures can use. They’re often found in low-nutrient waters like the Gulf of Mexico, making them an important part of the food chain, too.

While Chief Scientist Doug Capone (U.S.C.) is considered an expert on this organism, he and his colleagues admit there is much to be learned about Trichodesmium. On this voyage, Capone has brought researchers from his lab, like PhD student Laila Barada, to measure how much nitrogen the organisms fix, and study what nutrients they respond to.

“Tricho are very under-studied in this area,” Barada said. “What’s nice about finding it is that we haven’t found it here before.”

Barada and fellow PhD student, Amanda Liss, collect tricho by dragging a conically-shaped net behind the ship at various times of the day. Together with lab manager Brian Wilson (Eric Webb lab, U.S.C.), they carefully extract all the phytoplankton they’ve caught, pouring it into small bottles and looking for Trichodesmium by gently combing through the biomass.

Once they’ve separated all the tricho, Barada takes some of it inside where she does an experiment, called an acetylene reduction assay, that will help her measure nitrogen fixation. Since measuring actual nitrogen fixation is very difficult, she’ll use acetylene gas, which, like nitrogen, is a gas with a triple bond. The tricho will respond to it much like they would nitrogen gas, only this time they’ll produce ethylene, which is easier for Barada to measure. With her ethylene measurements, Barada can then interpolate how much nitrogen each organism might fix.

Wilson, meanwhile, takes a portion of the Trichodesmium they’ve gathered and takes it to his “dark room” to examine under a microscope. His job is to try and identify, among other phytoplankton species, all the tricho, and catalog it for the growing database.

Situated in the ship’s office, Wilson works at a small desk rigged with a microscope, lighting and a camera to photograph various specimen. His small workspace is further closed in by heavy blue curtains, hung to block out unnecessary light.

Once they get home, both Wilson and Barada’s labs will conduct DNA and RNA tests on the tricho to identify various Trichodesmium species, and to see if the organisms have fixed nitrogen recently.

Barada, Liss and Wilson have not found many tricho while on this voyage in the Eastern Tropical South Pacific, but they are not discouraged. There are simply too many variables—like weather, water temperature, and nutrient availability—and too little data to conclude that Trichodesmium can’t be found in abundance here, Barada said.

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